Photonic sensors and imaging systems rely on two dominant classes of technology to detect photons; the first, generally referred to as “photoelectric” typically employs diode-like structures which rely on semiconductor material energy band gaps to define wavelength ranges of interest. The second class of detectors are typically referred to as “thermal detectors.” The detectors typically detect incident radiation based upon the thermal heating of individual elements or pixels of an imaging array, wherein the absorbed radiation is converted to heat that subsequently produces a measured effect. Intrinsically, they tend to respond to all wavelengths. This class of photonic sensors includes bolometers, thermocouples, thermopiles, thermistors and pyroelectric materials, and generally makes use of materials that have one or more physical properties that respond in an exaggerated manner to changes in the temperature of that material. Although this class of sensors is responsive to photonic radiation of any wavelength, traditionally this class of sensors has been employed for the detection of infrared radiation, and consequently the elements of this class have been referred to as infrared sensors and imagers. They will be referred to as infrared sensors and imagers in this document.
The ability to accurately, reliably, and sensitively detect low levels of infrared radiation generally relies on the appropriate choice of sensor material properties. In particular, the heat capacity and the thermal conductance of each of the materials that comprise the sensor elements are carefully chosen, as are the thermal properties of adjacent materials that affect the flow of heat away from the sensing elements. For example, the thermal conductivity of air cannot be neglected, with the result that many thermal imaging systems are packaging in a vacuum enclosure. In order to obtain high detection sensitivity, it is generally desirable that the heat flow away from the individual sensor elements or pixels be relatively slow, and consequently efforts are made to minimize thermal conductance paths from the sensor elements to adjacent sensor elements and to substrate heatsinks. One of the means to achieve such reduced thermal conductance to the substrate is to employ the techniques of bulk or surface micromachining in order to remove substrate material from beneath the sensor elements so as to reduce the thermal conductance between the sensor element and the remaining substrate material. The thermal conductance of air, or of other gasses in this gap, is generally less than that of most solid materials. Further reductions in thermal conductance across this gap can be obtained by operating the sensing array in a vacuum environment. In particular, these techniques have been advantageously employed in the fabrication of arrays of microbolometers for imaging infrared radiation.
However, the speed of response of a thermal sensor is also related to the thermal conductance between the sensor element and the substrate or heatsink. A given sensor element can only respond to changes in incident radiation if the heat absorbed from the incident radiation can be dissipated to the substrate. Thus, in order to achieve adequate temporal response of a thermal sensor, it is desirable to increase the thermal conductance path between the active sensing element and the physical surrounding region or substrate.
Accordingly, it is desirable to provide a thermal imaging device or sensor element that address this simultaneous need for low thermal conductivity, in order to increase sensitivity, and high thermal conductivity, in order to increase sensor response speed.
Accordingly, it is desirable to provide an infrared imaging system, with a means for varying the paths of thermal conductance.